A density-sensing factor regulates signal transduction in Dictyostelium.

Abstract

Abstract. Dictyostelium discoideum initiates development when ceils overgrow their bacterial food source and starve. To coordinate development, the cells monitor the extracellular level of a protein, conditioned medium factor (CMF), secreted by starved cells. When a majority of the cells in a given area have starved, as signaled by CMF secretion, the extracellular level of CMF rises above a threshold value and permits aggregation of the starved cells. The cells aggregate using relayed pulses of cAMP as the chemoattractant. Cells in which CMF accumulation has been blocked by antisense do not aggregate except in the presence of exogenous CMF. We find that these cells are viable but do not chemotax towards cAMP. Videomicroscopy indicates that the inability of CMF antisense cells to chemotax is not due to a gross defect in motility, although both video and scanning electron microscopy indicate that CMF increases the frequency of pseudopod formation.The activations of Ca 2+ influx, adenylyl cyclase, and guanylyl cyclase in response to a pulse of cAMP are strongly inhibited in cells lacking CMF, but are rescued by as little as 10 s exposure of cells to CMF. The activation of phospholipase C by cAMP is not affected by CMF. Northern blots indicate normal levels of the cAMP receptor mRNA in CMF antisense cells during development, while cAMP binding assays and Scatchard plots indicate that CMF antisense cells contain normal levels of the cAMP receptor. In Dictyostelium, both adenylyl and guanylyl cyclases are activated via G proteins. We find that the interaction of the cAMP receptor with G proteins in vitro is not measurably affected by CMF, whereas the activation of adenylyl cyclase by G proteins requires cells to have been exposed to CMF. CMF thus appears to regulate aggregation by regulating an early step of cAMP signal transduction.

Involvement of Sib Proteins in the Regulation of Cellular Adhesion in Dictyostelium discoideum

“…The sibA mutants were described previously (12). The ampA mutant cells (2), countin mutant cells (17), smlA mutant cells (4), cmf mutant cells (25), cf50 mutant cells (5), and aprA mutant cells (6) were kind gifts of D. Blumberg (University of Maryland) and R. Gomer (Rice University).…”

A secreted factor represses cell proliferation inDictyostelium

“…When the amoebae are starved for bacteria, they cease dividing and begin secreting an 80 kDa glycoprotein called conditioned medium factor (CMF). When there is a high density of starving cells, as indicated by a high concentration of CMF (Jain et al, 1992;Yuen et al, 1995), the cells aggregate between 5 and 10 hours after starvation (Aubry and Firtel, 1999). The aggregating cells form large streams that break up into groups of ~20,000 cells (Shaffer, 1957).…”

Involvement of a Rab8-like protein of Dictyostelium discoideum, Sas1, in the formation of membrane extensions, secretion and adhesion during development

“…When cells reach high density and during early development, D. discoideum amoebae secrete lysosomal hydrolases (Cardelli, 1993;Temesvari et al, 1996), celldensity sensing factor (CMF) (Gomer et al, 1991;Jain et al, 1992;Yuen et al, 1995) and cell-counting factor (CF) (Brock & Gomer, 1999;Roisin-Bouffay et al, 2000;Brock et al, 2002;Jang et al, 2002;Tang et al, 2002). CF is a 450 kDa protein complex that can regulate aggregate size during development.…”